Turntable Belt Issue

I have a Systemdek 2x2 turntable. The rubber belt broke, and while its replacement seems to work fine at 33 1/3 rpm, it won’t stay on when I switch it over to portion of the motor pulley for 45 rpm playback. I tried stretching the belt out a bit as it seems to fit slightly tighter than the old one, but no luck. After a rotation or so at the 45 rpm, the new belt simply works its way off the sub-platter. Every time. Suggestions?

 

Is it wider than the original?

 

Its looks identical, it just seems to fit a bit tighter, which I suspect is the problem.

 

Perhaps let it run in for several hours?

 

Replace the platter before turning the motor back on. The belt will stay on in the 33 rpm position but not with the belt in the 45 rpm position unless the platter is installed.
-Bill

 

Is it a sprung suspension deck? That's the way it works on my sprung suspension Merrill, need to have the platter on the sub platter to get the motor and subplatter aligned at 45 rpm or the belt will slip off.

 

Yeah, it's basically an AR / Linn type set-up.
-Bill

 

The new belt would work its way off even if the platter was placed on the sub-platter. I sort of solved this issue- I took the belt off of an old Rega Planar 3 , put it on the systemdek, and it seems to work fine at both speeds. Thanks for the suggestions though!